This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems and, more particularly, to a method for reducing image artifacts caused by subject motion.
In at least some “computed tomography” (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as an “imaging plane”. The x-ray beam passes through a subject being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile, or “projection”.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units,” which are used to control the brightness of a corresponding pixel on a display.
To reduce the total scan time required for multiple slices, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
When an image is acquired of the beating heart or surrounding tissues, it is difficult to produce an image without artifacts. The cardiac cycle includes a period of relaxation and dilation of the heart cavities known as diastole, and a period of contraction of the heart during which blood is ejected from the ventricles known as systole. A typical period of time for one cardiac cycle is slightly less than one second. Thus, a heart goes through a substantial portion of its cycle during one gantry revolution. Motion induced image artifacts result from such heart motion. Known cardiac CT scanners utilize “electro-cardio-gram” (ECG) signals to gate the acquisition of scan data. Typically, leads are connected to a patient to measure the ECG signal, which indirectly represents a cardiac cycle.
With ECG cardiac gating the presumption is made that there is a direct and consistent correlation between the phases of the ECG periodic signal and the physical position and shape of the heart. Thus, if data is acquired only during a particular phase or phases of the periodic ECG signal, the assumption is that the heart will be in a particular position and shape when all the scan data is acquired. This assumption is not always correct. For instance, the heart rate typically changes during the injection of contrast agents that are required for imaging the coronary arteries. This creates real problems for ECG gating in that the fractions of the QRS interval that might successfully produce consistent data for static/gated images before injection would not be the same after contrast injection. There is no reason to believe that a given fraction of the ECG cycle would produce consistent reconstructions when the heart beat changes. At a minimum, more radiation will be required when the heart rate changes to acquire enough projections to permit adequate reconstructions. The situation is worse with arrhythmias. Depending on the arrhythmia, there may never be segments that are consistent from one ECG cycle to the next.